1. Field of the Invention
This invention is directed at a process for removing an acidic compound from a gaseous mixture. More particularly, this invention is directed to a process for removing CO.sub.2, from a gaseous mixture containing the CO.sub.2 by contacting the gaseous mixture with a two liquid phase scrubbing solution comprising an upper liquid phase comprising a sterically hindered amine-rich liquid stream having a relatively high alkali bicarbonate to carbonate ratio and a lower liquid phase having relatively low alkali bicarbonate to alkali carbonate ratio.
2. Description of the Prior Art
In petroleum and petrochemical processing, the feed-stocks often contain acidic compounds such as CO.sub.2, H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS, and the oxygen and sulfur derivatives of C.sub.1 to C.sub.4 hydrocarbons. Environmental regulations frequently limit emissions of the sulfur-containing compounds. It is also desirable to reduce the concentration of CO.sub.2 in the gaseous feedstocks, since CO.sub.2 may be corrosive to process equipment or it may be necessary to remove the CO.sub.2 to increase the concentration of the other components in the gas stream. In the manufacture of LNG it is necessary to remove CO.sub.2 down to very low levels to prevent the formation of solid CO.sub.2. Accordingly, the acidic compounds often are removed from gaseous feedstocks by passing the feedstock including the acidic compounds through an acid gas removal step.
Where the bulk removal of an acidic compound is desired and the feed gas pressures are relatively high, one process typically used is conventionally referred to as the "aqueous base scrubbing process" or "hot potash" process. In this process, the acid-containing feedstock is passed upwardly in an absorber while an acid-gas absorbing scrubbing solution containing an alkaline compound, such as a potassium compound, is passed downwardly. The scrubbing solution is then passed to a desorber, or regenerator, where at least a portion of the acidic compound is stripped off. Means for reducing the capital and operating costs of these systems recently have been directed at alkali metal scrubbing solutions having improved absorptive and desorptive capacities. U.S. Pat. No. 4,112,050, is directed at the discovery that the incorporation of a sterically hindered amine, i.e., an amine containing at least one secondary or tertiary carbon atom or a primary amine group attached to a tertiary carbon atom, improves the efficiency of the scrubbing process. U.S. Pat. No. 4,112,051 is directed at the use of a sterically hindered amine and a solvent for the amine. U.S. Pat. No. 4,094,957 teaches that when sterically hindered amines selected from the group consisting of aminoethers, amino-alcohols, di- and triamines are used in a "hot potash" CO.sub.2 containing acid gas scrubbing process, phase separation of the scrubbing solution often occurs under acid gas-lean conditions, particularly during desorption. Frequently, phase separation was found to occur when the rate of conversion of the potassium carbonate to potassium bicarbonate was less than about 30%.
Previously, it has been thought that phase separation should be avoided in scrubbing systems since this complicated the scrubbing system operation. Variations in the composition of the scrubbing solution recirculated to the absorber resulted in fluctuations in the concentration of acid gas remaining in the exiting treated gas. Moreover, phase separation also resulted in variations of the heat duty required as the composition in the reboiler varied. As a result, affirmative steps typically have been taken to prevent phase separation. These steps have included:
1. Incompletely desorbing the scrubbing solution to keep the scrubbing solution in the one phase region. However, this increases the capital and operating costs of the system; or PA1 (I) an alkaline material selected from the class consisting of alkali metal salts and alkali metal hydroxides; and PA1 (II) a sterically hindered amine to thereby at least partially absorb the carbon dioxide;
2. Adding a co-solvent, typically an amino acid having four to eight carbon atoms, to prevent phase separation. However, amino acids are relatively expensive, particularly when periodic addition of make-up scrubbing solution to the system is required.
Moreover, prior art scrubbing systems frequently do not reduce the concentration of the acidic compound in the treated gas to relatively low levels without excessive increases in the scrubbing solution circulation rate and in the reboiler duty.
It is, therefore, advantageous to utilize a scrubbing process which does not utilize cosolvents and which can be operated for substantially complete desorption of the scrubbing solution. It is also advantageous to provide a scrubbing process which reduces the acid concentration in the treated gas to sufficiently low levels.
It has been found that, where a scrubbing system comprising a sterically hindered amine and an alkali metal salt, or an alkali metal hydroxide is operated such that the at least partially desorbed solution forms a two phase liquid, addition of the two phases to selected spaced-apart locations in the absorber results in a reduced concentration of the acidic compound in the treated gas.